1. Field of the Invention
This invention relates to the art of refining particulate material and is more particularly concerned with refining paper making pulp stock.
2. Description of Prior Art
Mechanical wood pulp is initially reduced to a fibrous form by grinding logs on a rotating stone or by grinding chips in a disk mill. Following this initial fiberization, any further comminution of the mechanical wood fibers is accomplished in a disk mill, generally operated in the vapor phase using either pressurized or non-pressurized conditions.
The vapor phase milling or refining system suffers from two major drawbacks, namely, high energy consumption, and difficult system control. Energy application in a vapor phase system is quite inefficient and, thus, requires high energy consumption to achieve necessary freeness reduction in pulping quality development. The heterogenous nature of the milling process, coupled with the constantly changing condition of the refiner plates, means that energy application must be constantly altered to maintain a uniform freeness or pulp quality. In a typical vapor phase system, the inherent latency induced into the pulp must be removed prior to drainage measurements. This causes a lag time of, for example, 30 to 60 minutes in system feedback control. Thus it is very difficult to maintain a highly uniform product.
Low consistency refining as an alternative to vapor-phase refining would be much preferred. However, heretofore low consistency refining has proven ineffective because classical low consistency refining techniques typically used for chemical pulp fibers, were used for refining mechanical pulps. The resulting pulps exhibited severe fiber shortening with little or no strength development from increased bonding. The stiff, brittle nature of the mechanical pulp fibers requires that a low refining intensity be applied to the pulp to prevent fiber shortening. At the same time, a substantial amount of energy, by low consistency refining standards, must be applied to the fiber to generate the very high specific surface required in mechanical pulps. Existing refiners do not have the capability of providing low intensity and high specific energy at commercially acceptable throughputs.
Refining intensity is defined as horsepower per refiner bar inch crossings per minute (HP/ICPM) and specific energy is defined as horsepower per ton per day (HP/T/D). Therefore, to provide low refining intensities and high specific energy at commercial throughputs, an extremely large number of ICPM's are required within one machine. To provide this capability in a conventional single or double disk low consistency refiner would require prohibitively large refining disk diameters or rotational speeds.
An additional concern, in low consistency refining, is the fact that the rotating disks act as a hydraulic pump, resulting in large energy requirements for the refiner during the circulating or no-load condition. This circulating load increases proportionally with the cube of the speed, resulting in very high no-load energies at high RPM. Therefore, it is critical to keep the circulating energy as low as possible in proportion to the net refining energy.
By way of example, attention is directed to the following prior U.S. patents representing refiners which, however, do not attain the desired results for low consistency refining of mechanical pulp:
U.S. Pat. No. 3,371,873 discloses a single rotary disk arrangement and which inherently lacks the desired low consistency refining capability due to the ineffeciencies mentioned hereinabove.
U.S. Pat. No. 2,718,178 discloses a multi-disk arrangement wherein the disks are widely axially spaced in an arrangement which requires unacceptably large space for minimum refining results.
U.S. Pat. No. 4,167,250 discloses a multi-disk arangement requiring an unacceptably complex driving system.